Well, except for the fact that they ARE bolted to one another....
Not true. Harmonic vibrations can occur under a whole host of operating conditions, not just a specific RPM.
The engineers that developed the engine spent tens of thousands of hours doing NVH testing under a whole lot more widely varied operating conditions than your 2000-3000 RPM test, and determined that the balance shaft was necessary for some specific reason. It's likely that the extra weight/inertia/shape/polar moment of the transfer case added to the driveline, or possibly the front half of the driveline adversely affects the vibration of the engine, or the engine itself transmits vibrations in some undesirable way to some component (be it the engine itself, the mounts, the passenger compartment, or any of a thousand places) and tha undesirable vibration is unique to the 4x4 drivetrain in some way. At any rate, it was installed there for a reason, so unless you know something the engineers don't, I don't think you can call it a 'myth' in the first place, let alone 'debunk' it based on one instance.
The question was never whether or not it was possible to bolt a 2wd engine into a 4x4 truck. They are the same engine, aside from the balance shaft. The question is whether or not doing so would have any negative side effects. One can only infer from the original design that there was some specific reason the engineers opted to install the balance shaft on ONLY the 4x4 vehicles, and one can logically infer that the engineers would not have insisted on installing it if there wasn't a need for it. Should you choose to do otherwise, then it must mean you are much wiser than all the engineers that designed, tested, and developed that engine.... And that's a pretty bold claim, IMHO. I guess nearly 20 years of powertrain development must be wrong....
Let's not overlook the fact that it would appear that now ALL of the SOHC motors include the balance shaft, regardless of whether it's a 2wd or a 4x4. I didn't check to see exactly when that change occurred, but the service manuals for the '09 no longer specify the balance shaft for 4x4s only.... Again, if there was no benefit derived from it, they would have gone the other way and removed it from all the engines.